What do you mean by, "any group that can't clear ambush also won't have enough damage to kill the summon in the first place." We each take it down a couple layers before the ambush, won't change after the ambush unless we can't defeat the * ambush ninjas.
And, since my group doesn't really have that many players active for group summon we can't form teams to beat them. It's pointless for smaller groups too if the members level difference is dramatic and the ninjas do a lot of damage, just like my group.